First quantum transmission sent through space

Secret encryption keys transmitted via quantum links provide the ultimate way to communicate securely. That’s because any attempt to intercept the key will be revealed thanks to the laws of quantum mechanics, which say that interception will introduce changes that give away eavesdroppers.

The technology is already available for fibre-optic cables, but a truly global network would need satellites to beam quantum data between distant locations. To test how these might work, Paolo Villoresi at the University of Padua in Italy and his colleagues turned to satellites covered in ultra-reflective mirrors. These are normally used to bounce laser beams back to Earth. The time they take to return shows up any shifts in gravity.

The team prepared photons in four different quantum states – the minimum required to generate an encryption key – and sent them to space using the Matera Laser Ranging Observatory in Italy.

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They were able to receive quantum bits of information, or qubits, bounced back from five satellites up to 2600 kilometres away. That smashes the previous record for sending quantum information – 144 kilometres, between two locations on Earth.

To correctly identify the photons carrying qubits, the team timed the laser pulses exactly, rejecting any light that returned outside a narrow time window.

The observatory acted as both transmitter and receiver, so no encrypted message was sent. Still, the team thinks the photons would have been able to encrypt about a bit of data per second (arxiv.org/abs/1406.4051).

Jian-Wei Pan at the University of Science and Technology of China in Hefei is not sure the signal would be strong enough to transmit a quantum key, but says the work shows the technology is getting close.

This article appeared in print under the headline “Quantum bits make first leap through space”